Cold-formed spring having high fatigue strength and high corrosion fatigue strength, steel for such spring, and method of manufacturing such spring

ABSTRACT

The present invention provides a cold-formed spring having high fatigue strength and high corrosion fatigue strength, a specific type of steel for such a spring, and a method of manufacturing such a cold-formed coil spring. The spring according to the present invention is made from a steel material containing, in weight percentage, 0.45 to 0.52% of C, 1.80 to 2.00% of Si, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to 0.30% of V, with Fe substantially constituting the remaining percentage. A wire is produced from the steel, and the wire is subjected to a high-frequency heating process, whereby the wire is hardened at a temperature of 920 to 1040° C. for 5 to 10 seconds, and then tempered at a temperature of 450 to 550° C. for 5 to 20 seconds so that its hardness becomes 50.5 to 53.5 HRC. Finally, the wire undergoes a shot peening process so that its residual stress at 0.2 mm depth from the surface becomes −600 MPa or higher.

This is a Division of application Ser. No. 10/823,629 filed Apr. 14,2004, which claims the benefit of Japanese Patent Application No.2003-114829 filed Apr. 14, 2003. The disclosures of the priorapplications are hereby incorporated by reference herein in theirentirety.

The present invention relates to a cold-formed spring having highfatigue strength, a type of material for such a spring, and a method ofmanufacturing such a spring. More specifically, the present inventionrelates to a cold-formed spring that must have high fatigue strengthagainst corrosive environments, e.g. a suspension spring used inautomobiles, and also relates to a type of material for such a springand a method of manufacturing such a spring.

BACKGROUND OF THE INVENTION

For the purpose of environmental protection and resource conservation,it is low demanded that the amount of harmful substances contained inthe exhaust gas emitted from automobiles should be reduced, while it isalso desired that automobiles should have better fuel efficiency. Tomeet such demands, one effective measure is to make the body of theautomobile lighter. Accordingly, efforts have been made to make everypart of the body as light as possible.

An example of such a body part is the suspension spring, which willcontribute to the production of a lightweight body if it has a higherworking stress (or design stress). An improvement of the working stress,however, may cause a problem in respect of the fatigue (or durability)of the spring.

Another problem is the corrosion of the spring, which is unavoidablebecause suspension springs are installed in such locations of the bodythat are most badly stained with water or mud. Corrosion creates pits(or micro-pores) on the surface of the spring, and these pits serve asthe starting point for the fatigue fracture of the spring.

To address the aforementioned problems, the applicant has filed aJapanese patent application for a “spring having an improved corrosionfatigue resistance”, as disclosed in the Japanese Unexamined PatentPublication No. H11-241143.

The aforementioned spring exhibits high durability even under highworking stress. In the development of this spring, however, it wasassumed that the spring would be hot-formed. If, as in the presentinvention, the spring is used as a cold-formed material, the spring mayhave a poorer durability because of an excessive decarburization (i.e.,a phenomenon in which carbon content escapes from the surface of thespring when the spring is heated at high temperature).

Accordingly, what remains unsolved is to obtain a specific type of steelfor cold-formed springs that has good durability (or fatigue resistance)as well as good corrosion resistance, and a cold-formed coil spring madefrom the steel.

In view of the aforementioned problems, the present invention intends toprovide a cold-formed spring having high fatigue strength (i.e. fatigueresistance or durability) and high corrosion fatigue strength, aspecific type of steel material for such a spring, and a method ofmanufacturing such a cold-formed coil spring.

SUMMARY OF THE INVENTION

To address the aforementioned problems, the present invention provides acold-formed spring having high fatigue strength and high corrosionfatigue strength, which is made of a wire made from a steel materialcontaining, in weight percentage, 0.45 to 0.52% of C, 1.80 to 2.00% ofSi, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to 0.30% of V,with Fe substantially constituting the remaining percentage, and whichis hardened and tempered by a high-frequency heating process.

In the aforementioned steel material, it is preferable that thepercentage of P is 0.025% or lower and the percentage of S is 0.020% orlower.

It is also preferable that the wire has the tensile strength of 1800 to2000 MPa and a reduction of area of 35% or higher after being hardenedand tempered by the high-frequency heating process.

It is also preferable that the wire has a hardness of 50.5 to 53.5 HRCafter being hardened and tempered, and the spring is subject to a shotpeening process so that the residual stress at 0.2 mm depth from thesurface becomes −600 MPa or higher.

The present invention also provides a method of manufacturing a coilspring having high fatigue strength and high corrosion fatigue strength,in which the spring is made from a steel material containing, in weightpercentage, 0.45 to 0.52% of C, 1.80 to 2.00% of Si, 0.30 to 0.80% ofNi, 0.15 to 0.35% of Cr and 0.15 to 0.30% of V, with Fe substantiallyconstituting the remaining percentage, and which includes the steps ofmaking a wire from the steel material, hardening and tempering the wireby a high-frequency heating process and cold-coiling the wire into thespring.

It is preferable that the high-frequency heating process includes thesteps of hardening the wire at a temperature of 920 to 1040° C. for 5 to20 seconds, rapidly cooling the wire, and tempering the wire at atemperature of 450 to 550° C. for 5 to 20 seconds. More preferably, thehardening temperature is within the range from 940 to 1020° C. and thetempering temperature is within the range from 480 to 520° C.

It is also preferable that the wire is rapidly cooled after beingtempered.

The present invention also provides a type of steel material forcold-forming a spring hardened and tempered by a high-frequency heatingprocess, which contains, in weight percentage, 0.45 to 0.52% of C, 1.80to 2.00% of Si, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to0.30% of V, with Fe substantially constituting the remaining percentage.

In the steel material, it is preferable that the percentage of P is0.025% or lower and the percentage of S is 0.020% or lower.

For the cold-formed spring having high fatigue strength and highcorrosion fatigue strength according to the present invention, thepercentage ranges of the elements of the steel material have beenspecified on the basis of the following reasons.

Carbon (C): 0.45 to 0.52%

Carbon has the greatest influence on the strength of the steel material,and any steel material for suspension spring must contain 0.45% or moreof carbon to have such a strength that provides an adequate durability(or fatigue resistance). However, when the carbon content is higher than0.52%, the corrosion fatigue strength will deteriorate due to thedecrease in the toughness of the material.

Silicon (Si): 1.80 to 2.00%

Similar to carbon, silicon increases the strength of the steel material.Also, in the case of manufacturing a spring, silicon is an importantelement to increase the sag resistance of the spring. Under normalworking conditions for automobiles, the sag of the spring will benoticeable when the silicon content is lower than 0.18%, which maydecrease the height of the body. Silicon also promotes the surfacedecarburization during the heating process. For a spring that ismaximally loaded on its surface when used, the decarburization must beprimarily considered. When the silicon content is higher than 2.0%, thedecarburization will be noticeable during the heating process forhardening. For this reason, the present invention has set the upperlimit of the silicon content at 2.0%.

Nickel (Ni): 0.30 to 0.80%

Nickel improves the corrosion resistance of the steel material. In thecase of a suspension spring, the nickel content must be 0.30% or higherto provide an adequate corrosion resistance. Use of more than 0.80% ofnickel, however, is not recommendable because it provides no improvementof the corrosion resistance, which is saturated at 0.80%, while itunnecessarily increases the manufacturing cost due to nickel being anexpensive element.

Chromium (Cr): 0.15 to 0.35%

Similar to nickel, chromium improves the corrosion resistance of thesteel material. Furthermore, chromium improves the hardening effect. Toprovide the steel material with adequate strength, toughness anddurability, the heating process must be fully performed. Therefore, thespring must be completely hardened to its core. For this purpose, thesteel material according to the present invention contains 0.15% or moreof chromium. However, with respect to the diameter of the suspensionspring that the present invention concerns, 0.35% of chromium provides asufficient hardening effect. Percentages higher than that willundesirably increase the residual austenite.

Vanadium (V): 0.15 to 0.30%

Vanadium precipitates in the form of fine particles of carbide insidethe steel material, which prevents the development of crystal grainsduring the heating process. The reduction of the grain size is effectivein improving the corrosion fatigue resistance as well as the toughnessof the steel material. To obtain such effects, the vanadium content mustbe 0.15% or higher. The percentage, however, needs to be 0.30% or lowerbecause percentages higher than that are likely to promote thedevelopment of each vanadium carbide particle rather than increase theprecipitation sites of the carbide. The development of vanadium carbideparticles may decrease the toughness and the corrosion fatigueresistance.

Phosphorus (P): 0.025% or lower

Phosphorus is the first element to precipitate within the grain boundaryinside the steel material and deteriorates the strength of the grainboundary. Since the precipitation of phosphorus decreases the fatiguestrength, it is desirable to make the phosphorus content as low aspossible. With respect to the process capability of the manufacturingprocess and the prescribed properties of the spring, the phosphoruscontent should be preferably 0.025% or lower.

Sulfur (S): 0.020% or lower

Inside the steel material, sulfur is combined with manganese into MnS,which is insoluble into the steel material. Since MnS is a softsubstance, it is easily extended through rolling or a similar process,which deteriorates the mechanical properties of the steel material.Therefore, in manufacturing the spring, it is preferable to make thesulfur content as low as possible. With respect to the processcapability of the manufacturing process and the prescribed properties ofthe spring, the sulfur content should be preferably 0.025% or lower.

A typical process of manufacturing a cold-formed spring includes thefollowing steps: rolling a material into a wire; changing the diameterof the wire to a predetermined value by drawing or a similar process, ifnecessary; hardening and tempering the wire; coiling the wire into aspring; and conducting the shot peening and the pre-setting.

The cold-formed spring according to the present invention ismanufactured by using a specific type of steel material whosecomposition satisfies the above-described conditions, and controllingthe hardening and tempering process so that the hardness of the springbecomes 50.5 to 53.5 HRC. When the hardness is lower than this range,the spring cannot have sufficient durability (or fatigue-resistance) tobe used as a suspension spring. When the hardness is higher than therange, the cold-coiling of the wire will be difficult and the coilingprocess will cause some quality damages of the spring, such as a surfaceflaw, surface crack or the deterioration of the toughness due to anexcessive working effect.

According to the present invention, the hardening and tempering isaccomplished by a high-frequency heating process. The high-frequencyheating makes it possible to rapidly raise the temperature and minimizethe surface decarburization. This heating process is also advantageousin that the crystal grains inside the steel material have little time todevelop. Furthermore, this heating process provides a relatively easycontrol of the temperature with good accuracy. These effects areadvantageous particularly for the hardening process. In the temperingprocess, it is also preferable to use a slightly high temperature toshorten the processing time to obtain the same effect (i.e. temperhardness). This preferably improves the sag resistance of the spring.

For example, it is preferable that, in the high-frequency heatingprocess, the steel material is hardened at a temperature of 920 to 1040°C. (more preferably 940 to 1020° C.) for 5 to 20 seconds, then rapidlycooled, and finally tempered at a temperature of 450 to 550° C. (morepreferably 480 to 520° C.) for 5 to 20 seconds. The temperaturesspecified hereby are higher than in the case of the normal furnaceheating and accordingly shorten the heating time (or heat-up time),whereby the decarburization, the development of the crystal grains andsome other problems are prevented.

Rapid cooling after the tempering is also recommendable because itreduces the unevenness in temper hardness.

According to the present invention, the conditions for the shot peeningprocess is regulated so that the residual stress becomes −600 MPa orhigher at 0.2 mm depth from the surface. Given this level of compressionresidual stress at the surface, the spring will have an adequatedurability as a suspension spring. The shot peening may be performedeither under cold temperature (at room temperature) or warm temperature(at about 250 to 340° C.).

As described above, the cold-formed spring according to the presentinvention is manufactured by preparing a steel material having aspecific composition and performing a high-frequency heating processunder specific conditions. The spring thus manufactured has goodcorrosion fatigue resistance to be used as a suspension spring. Theappropriate determination of the conditions for the heating, shotpeening and other subsequent processes minimizes the amount of sag thatmay arise when the spring is used. Furthermore, the coiling work isfacilitated and the quality deterioration due to the coiling work isminimized.

Having the above-described good properties, the cold-formed coil springaccording to the present invention can be used under a maximum designstress of 1150 MPa or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a table showing the evaluation result of the decarburizationproperty with respect to the carbon content and the silicon content.

FIG. 2 is a graph showing the optimal region of the carbon content andthe silicon content.

FIG. 3 is a graph showing the relation between the carbon content andthe corrosion durability.

FIG. 4 is a graph showing the relation between the nickel content andthe weight loss through corrosion.

FIG. 5 is a graph showing the relation between the vanadium content andthe crystal grain size number.

FIG. 6 is, a graph showing the relation between the phosphorus contentand the corrosion durability.

FIG. 7 is a graph showing the relation between the tensile strength andthe reduction of area of an example of the steel material according tothe present invention and a comparison steel material.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Examples of the present invention are described with reference to thedrawings.

Twenty pieces of steel samples having different carbon and siliconcontents were prepared for an investigation of the decarburizationproperty. Each sample was heated for 10 minutes up to 900° C., which wasthen rapidly cooled and cut. The cut surface was observed with amicroscope, and the sample was evaluated as either “OK (Good)” (when thedepth of the perfect (ferrite) decarburization layer was less than 0.02mm) or “NG (No Good)” (when the depth was 0.02 mm or more). The resultis shown in FIG. 1.

From FIG. 1, the optimal region of the carbon content and the siliconcontent with respect to the decarburization has been determined, asshown in FIG. 2. The optimal region corresponds to 0.45 to 0.52% ofcarbon content and 1.80 to 2.00% of silicon content, in weightpercentage.

In FIG. 2, the steel lacks strength within the region with the siliconcontent lower than 1.80% and the carbon content lower than 0.52%. Inthis region, the lack of durability causes a considerable amount of sagwhen the steel is used as a spring. In the region with the siliconcontent higher than 2.00%, the decarburization is undesirable. In thisregion, the surface strength of the steel may significantly decrease dueto the decarburization during the heating process. In the region withthe carbon content higher than 0.52%, the steel lacks toughness. When,as in the case of the suspension spring, the steel is used under a verycorrosive environment, the lack of toughness causes a decrease in thedurability.

The second experiment focused on the relation between the carbon contentand the corrosion durability. In this experiment, the loading conditionwas 490±294 MPa. The contents of the principal elements other thancarbon were as follows: Si: 1.99%, Mn: 0.69%, Ni: 0.55%, Cr: 0.20% andV: 0.20%. The result of this experiment is shown in FIG. 3.

FIG. 3 shows that the number of cycles to failure in the corrosiondurability test is greater than 50,000 when the carbon content is 0.52%or lower, meaning that the corrosion durability is adequate. When thecarbon content is higher than 0.52%, the number of cycles to failurerapidly decreases to about 30,000 or less.

The third experiment focused on the relation between the nickel contentand the corrosion resistance. The contents of the principal elementsother than nickel were as follows: C: 0.49%, Si: 1.99%, Mn: 0.69%, Cr:0.20% and V: 0.20%. In the experiment, the process of spraying a salinesolution at the temperature of 35° C. onto the sample for 3 hours anddrying the sample for 21 hours at the temperature of 35° C. was repeatedtwenty times. Upon completion, the weight loss through corrosion perunit surface area (kg/m²) was checked as the criteria for evaluating thecorrosion resistance. The result is shown in FIG. 4.

FIG. 4 shows that the weight loss is 0.4 kg/m² when the nickel contentis 0.30% or higher, meaning that the corrosion resistance is adequate.

The fourth experiment focused on the relation between the vanadiumcontent and the grain-refining effect. The contents of the principalelements other than vanadium were as follows: C: 0.49%, Si: 1.99%, Mn:0.69%, Ni: 0.55% and Cr: 0.20%. The result is shown in FIG. 5.

FIG. 5 shows that the crystal grain size number is greater than 10 whenthe vanadium content is within the range from 0.15 to 0.30%, meaningthat the grain-refining effect is adequate.

The fifth experiment focused on the relation between the phosphoruscontent and the corrosion durability. The contents of the principalelements other than phosphorus were as follows: C: 0.49%, Si: 1.99%, Mn:0.69%, Ni: 0.55%, Cr: 0.20% and V: 0.20%. The result is shown in FIG. 6.

FIG. 6 shows that the number of cycles to failure in the corrosiondurability test is greater than 50,000 when the phosphorus content is0.025% or lower, whereas the number decreases to about 20,000 or lesswhen the phosphorus content is higher than 0.025%.

The sixth experiment focused on the relation between the tensilestrength and the reduction of area of the wire made from a steelmaterial containing 0.49% of C, 1.99% of Si, 0.69% of Mn, 0.55% of Ni,0.20% of Cr and 0.20% of V. The wire was hardened by a high-frequencyheating process and then tempered at various temperatures so that itstensile strength becomes 1800 to 2000 MPa. The relation is shown in FIG.7, which also shows the property data of a conventional steel material(SAE9254) for comparison. The graph in FIG. 7 clearly shows that thesteel material according to the present invention has a higher ductilitythan the conventional one. This result suggests that the presentinvention improves the corrosion fatigue resistance.

1. A method of manufacturing a coil spring having high fatigue strengthand high corrosion fatigue strength, wherein the spring is made from asteel material containing, in weight percentage, 0.45 to 0.52% of C,1.80 to 2.00% of Si, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15to 0.30% of V, with Fe substantially constituting the remainingpercentage, and the method comprises the steps of making a wire from thesteel material, hardening and tempering the wire by a high-frequencyheating process and cold-coiling the wire into the spring.
 2. The methodaccording to claim 1, wherein the high-frequency heating processincludes the steps of hardening the wire at a temperature of 920 to1040° C. for 5 to 20 seconds, rapidly cooling the wire, and temperingthe wire at a temperature of 450 to 550° C. for 5 to 20 seconds.
 3. Themethod according to claim 2, wherein the hardening temperature is withinthe range from 940 to 1020° C. and the tempering temperature is withinthe range from 480 to 520° C.
 4. The method according to claim 2,wherein the wire is rapidly cooled after being tempered.